1. Field of the Invention
The present invention relates to a compositon having excellent water resistant properties, and more particularly to a water resistant composition comprising (A) polyvinyl alcohol (B) a chitosan of which aqueous 1% solution has a viscosity not greater than 70 centipoise (abbreviated as cp hereinafter) and (C) an aldehyde compound.
The present invention also relates to a heat-sensitive recording sheet, and more particularly to a heat-sensitive recording sheet having excellent water and solvent resistant properties, as well as improved color forming properties, which comprises a support having provided thereon with a heat-sensitive color forming layer, wherein (A) polyvinyl alcohol, (B) a chitosan of which aqueous 1% solution has a viscosity not greater than 70 cp at 20.degree. c. and (C) an aldehyde compound are incorporated into, or applied onto, said heat-sensitive color forming layer.
2. Description of the Prior Art
Polyvinyl alcohol has been widely used as binders, adhesives or sizing agents for treating surfaces. It is known that polyvinyl alcohol has film-forming properties and strength which are far superior to other sizing agents, but has relatively poor resistance to water, in particular, when dried at a low temperature, because it is essentially water soluble. Various methods have been attempted to improve the water resistant properties of polyvinyl alcohol.
One known method is to cross-link polyvinyl alcohol by aldehydes, such as glyoxal, glutaraldehyde and dialdehyde starch. However, in order to make polyvinyl alcohol sufficiently resistant to water by the use of such aldehydes, the polymer must be treated at an elevated temperature, e.g., as high as 100.degree. C., or even at a temperature higher than 120.degree. C. for a prolonged period of time. The cross-linking of polyvinyl alcohol can be effected at a relatively low temperature if polyvinyl alcohol is dried under a strongly acidic condition, e.g., not greater than pH 2.0. In this case, however, aqueous solution of polyvinyl alcohol tends to be unstable and suffers from gelation during actual usage.
It is also known to cross-link polyvinyl alcohol by the use of tetraborates of alkali metals. Cross-linked polyvinyl alcohol obtainable by this method shows only poor resistance to water and its aqueous solution has poor stability in viscosity.
In another known method, a water insoluble film is formed from polyvinyl alcohol and an acidic acetic acid solution of a chitosan having a high polymerization degree, or from a dilute solution of polyvinyl alcohol, a chitosan having a high polymerization degree and glutaraldehyde, and the resulting film is then dipped into a strong aqueous alkali solution, thereby hydrolyzing the polyvinyl alcohol to a high degree and, at the same time, converting the ammonium groups in the chitosan, which are formed by the reaction with acetic acid, into amines, so as to make the chitosan insoluble to water (see Proc. Int. Conf. (1982), T. Uragami et al., pp. 221-226). However, this method is basically developed for preparing a membrane for dialysis in a laboratorial scale and not suited for a commercial production. This is because chitosans produced by conventional methods have a high polymerization degree or a high molecular weight in the order of hundreds of thousands, that is to say, an aqueous 1% solution of the chitosans has a viscosity greater than 500 centipoise (cp) when measured by a Brookfield viscometer at 20.degree. C., and can be handled only in the form of a dilute solution since the viscosity of their aqueous solutions become too high when dissolved together with polyvinyl alcohol. In addition, ordinary chitosans have poor compatibility with polyvinyl alcohol and must be treated with alkali in order to be made insoluble to water.
On the other hand, heat-sensitive recording sheets comprising a support provided thereon with a colorless or lightly colored leuco dye, an electron acceptor such as a phenol compound or an organic acid, and a binder, have been widely used for facsimiles, computer terminals, printers for desktop electronic calculators, medical measuring instruments, bar code labels for POS, and the like. It is known that such heat-sensitive recording sheets can be produced by coating on a support a colorless or lightly colored lactone, such as Crystal Violet Lactone, and an acidic compound, such as a phenol compound, together with a water soluble binder.
As a water soluble binder for such heat-sensitive recording sheets, there have been used such water soluble polymers as polyvinyl alcohol, methyl cellullose, carboxymethyl cellulose, polyacrylic acid, copolymer of styrene and maleic aicd, starch, and the like. Of these polymers, polyvinyl alcohol has been most widely used because of its superior adhesive properties.
However, heat-sensitive recording sheets produced by using such water soluble binders suffer from the disadvantage that colored images formed by the intermixing and the reaction between a color-forming lactone and an acidic compound, tend to be faded or even disappeared when contacted with water or plasticizers contained in vinyl chloride polymers, e.g., for synthetic leathers, because of their instability to moisture and organic solvents.
It has been proposed to use, in order to overcome such a disadvantage, a water soluble binder together with an agent capable of imparting water resistant properties to the binder, including formalin, glyoxal, dialdehyde starch, glutaraldehyde, polyacrylamides modified with aldehydes, alkali metal salts of tetraboric acid, etc., as described in Japanese Patent Publication Nos. 33,117/83; 33,119/83; 33,839/83 and 30,555/84. However, it is not possible to attain satisfactory results by this method since such heat-sensitive recording sheets must be subjected, in order to make a water soluble binder, in particular polyvinyl alcohol, sufficiently resistant to water, to a heat treatment at an elevated temperature as high as 100.degree. C. or higher than 100.degree. C., whereas color forming reactions between the lactone compound and the acidic compound contained in the heat-sensitive sheets take place when heated at such a high temperature.
It has also been proposed to improve the water resistant properties of polyvinyl alcohol by imparting the capability of cross-linking to polyvinyl alcohol per se, as disclosed in Japanese Patent Application Laid Open Nos. 191,194/83 and 193,189/83. This method could be advantageous in that no heat treatment at a high temperature is required. However, heat-sensitive recording sheets obtainable by this method do not satisfy water and solvent resistant properties required for those to be used under severe conditions, e.g., for the POS, which has been introduced in the recent years.